Deployment Method of the Modbus Master-Slave Mode
The Modbus communication protocol is typically deployed in a master-slave configuration, where one device acts as the master and other devices serve as slaves. Within a network, the Modbus protocol exchanges information through a request-response mechanism between the master device (client) and slave devices (server). The master initiates requests, while slaves respond to them.Below is the standard deployment approach for the Modbus master-slave mode and its support for PLC devices:
Deployment Method:
1.Master:
1) The master is typically a control system, monitoring system, SCADA system, or other control device.
2) Only the master can initiate communications and send Modbus requests to slaves, such as reading or writing register values.
3) Only one primary station can be connected to the network at any given time.
4) The master station can use dedicated addresses to access each slave station individually, or use address 0 to access all slave stations simultaneously.
2.Slave:
1) A slave refers to the actual control device, such as sensors, actuators, PLCs, etc.
2) The slave station is responsible for responding to the master station's Modbus requests by providing the required data or executing the corresponding operations.
3) In a standard Modbus network, there is one master station and up to 31 slave stations.
4) The slave station cannot initiate communication with the master station or other slave stations.
3.Communication Protocol:
Modbus can operate across various physical layers, including serial ports (e.g., RS-232, RS-485) and Ethernet. The choice of physical layer depends on the specific application scenario and the distance between devices.
4.Communication Modes: The Modbus protocol supports two communication modes for information exchange:
1) Unicast mode – the master station accesses the slave station via a dedicated address; the slave station processes the request and responds to the master station.
2) Broadcast Mode – The master station accesses all slaves via address 0. In this mode, slaves do not respond to broadcast messages. (Therefore, request commands in broadcast mode must be write commands defined in the Modbus standard.)
5.Address Configuration:
Messages exchanged via the Modbus protocol contain the addresses of the data to be processed. Each slave in the Modbus network has a unique address; each register has a register number; and each data type (bit or register) has a 16-bit address. The master station uses these addresses to locate and communicate with slaves.
Typically, the address of the slave station is configured in the settings of the PLC device or other Modbus devices.
6.Protocol
1)Modbus communication employs distinct function codes (Function Codes) to denote various operations, such as reading registers and writing registers.
2) Both the master and slave stations must adhere to the same Modbus protocol specifications during communication.
Network topology supporting PLC devices:
1.Modbus RTU.
Modbus RTUs typically operate via RS-485 serial communication links, and the RS-485 bus wiring specification mandates a bus topology. In practical installations, a hand-in-hand daisy-chain wiring configuration is commonly employed. For example, three RS-485 devices (Slave 1, Slave 2, and Slave 3) are connected as follows: Slave 1's 485+ port connects to Slave 2's 485+ port, which then links to Slave 3's 485+ port, and so forth. The 485-port connections follow the same pattern as the 485+ ports.
2.ModbusTCP
Compared to Modbus RTUs that use serial links, Modbus TCP relies on Ethernet communication, offering greater flexibility in network topology. It has evolved from a master-slave configuration over serial links to a multi-client/multi-server architecture model.
Using Modbus TCP, the master device (client) can locate the Modbus slave device (server) via its IP address and connect to another Modbus RTU network through a Modbus gateway.
Support for PLC devices:
Modbus widely supports PLC devices. Many PLC devices inherently support the Modbus communication protocol, as Modbus is a universal and widely used industrial communication protocol.
Modbus-compatible PLC devices can function as slaves to communicate with other Modbus-compatible devices, or as master stations to communicate with other slaves, enabling data exchange and control among devices.
In specific industrial automation systems, the deployment of the Modbus master-slave configuration varies depending on the system architecture, device types, and communication requirements. Typically, the master station serves as a higher-level control system, while the slave stations can consist of various control devices, including PLCs. The flexibility of Modbus makes it one of the most widely used communication protocols in industrial control applications.